Extraction of petroleum (e.g., oil) is the process by which usable petroleum is drawn out from beneath the earth's surface. The amount of oil that is recoverable is determined by a number of factors, including the permeability of the rocks, the strength of natural drives, porosity of the reservoir rock, and the viscosity of the oil. When the reservoir rocks are “tight” such as in shale, oil generally cannot flow through, but when they are permeable such as in sandstone, oil flows freely.